Screening is the process of rendering the illusion of continuous-tone pictures on displays that are only capable of producing digital picture elements. In the process of printing images, large gray levels of the input picture have to be simulated by the printing device to reproduce a perfect duplicate of the original image. However, in the printed image the pixel resolution can be limited to that which is perceivable by the eye. Hence by grouping the adjacent pixels it is possible to simulate a continuous tone in the image.
Screening may take place by a threshold method in one of two categories: bi-level threshold screening; and multi-level threshold screening. In bi-level threshold screening the (x,y) coordinates of the input pixel are used to index into a screen cell. This is typically a two dimensional m by n matrix. The individual entries in the screen cell are gray level thresholds which are compared against the input pixel gray level. A binary value (0 or 1) is output based on the results of the comparison. Multi-level screening indexes into a three dimensional look-up table. This three dimensional look-up table is typically organized as a two dimensional screen cell of size m by n. The screen cell is a repeatable spatial tile in the image space. Each entry of the screen cell has a number of the tone curve which has to be used for the position of (x,y). The tone curve is the compensation transfer function of the input pixel gray value range to within range of the printing process. The tone-curve transfer function is quantized based on a set of thresholds and stored in the form of look-up tables. The look-up tables each contain 2b entries for an unscreened input pixel of size b-bits. All the 2b entries contain the corresponding screened output pixel of size c-bits. This process provides a manner of translating the color range of the input image into the smaller palette of the printer by mixing colors within the printer palette.
Screening in printing enables the illusion of continuous color or gray scale variations within an image using a limited palette of colors available to the printer. Traditional look-up table (LUT) based screening suffers from two problems. Look-up tables require a lot of storage space. Look-up tables also require a lot of bandwidth to access entries from external memory.